Types of a position encoder include an incremental type with which only relative displacement is detected and an absolute type with which an absolute position is detected. Of these, the incremental type photoelectric position encoder has no detection ability on positional displacement in a lateral direction perpendicular to a pitch direction (measurement direction). Thus, there was no need to control an offset in the lateral direction (i.e., an amount of positional displacement between a scale and a light receiving element in the lateral direction perpendicular to a relative movement direction of the scale and a detector for detecting positions) upon the assembly thereof.
As a position encoder used in a machine tool or the like for a feedback purpose, on the other hand, the absolute type position encoder capable of detecting an absolute position when a power is turned ON is typically employed (such an encoder is referred to as an “absolute encoder”). The absolute encoder includes two or more tracks in the lateral direction perpendicular to the measurement direction. Thus, displacement in the lateral direction generates signal crosstalk. As a result, an acceptable range of absolute position synthesis is reduced. This leads to a reduction in the reliability of absolute position synthesis. As a result, an absolute position cannot be detected. It is therefore necessary to adjust the positional relationship between the scale and the detector in an accurate manner.
A transmissive type photoelectric position encoder, in particular, includes: a light source 6 having a collimator lens 8, for example; a scale 10 having three rows of tracks 11, 12, and 13, for example; and a detector 20 having three light receiving elements 21, 22, and 23, for example, corresponding to the tracks 11, 12, and 13, respectively, as illustrated in FIG. 1 as an example. Since these components are aligned on the same optical axis, it is difficult to visually perform positional alignment between the scale 10 and the light receiving elements 21, 22, and 23.
In a case of an absolute encoder having a three-track configuration, for example, a scale 10 (FIG. 2B) includes an incremental track 11 (referred to as an “INC track”), an absolute narrow pitch track 12 (referred to as an “M1 track”), and an absolute wide pitch track 13 (referred to as an “M2 track”) and a detector 20 (FIG. 2A) includes light receiving element arrays 21 to 23 corresponding to the tracks 11 to 13, respectively, as shown in FIGS. 2A and 2B. In FIG. 2A, reference numerals 31 to 33 denote amplifiers; reference numeral 40 denotes a signal processor for performing analog-digital conversion and/or position calculation; reference numeral 42 denotes a counter device; and reference numeral 44 denotes a display device.
In order to detect a position appropriately with the configuration as shown in FIGS. 2A and 2B, it is required that the tracks 11 to 13 in the scale 10 and the light receiving element arrays 21 to 23 in the detector 20 be faced each other in a correct state as shown in FIG. 3. In contrast, if the positional relationship between the scale 10 and the detector 20 is misaligned, for example, in a lateral direction (in an upper direction in FIG. 4) as illustrated in FIG. 4 as an example, the light receiving element arrays 21 to 23 cannot detect a correct position.
According to the conventional position encoder, however, the light receiving element arrays 21 to 23 for obtaining main signals have no detection ability about displacement in the lateral direction. The conventional position encoder therefore cannot detect a lateral offset.
In view of the above, particularly in the separate type position encoder having the scale and the detector provided separately, means as will be described below have been proposed in order to set a positional relationship between the scale and the detector in a correct manner when installed in a device. For example, the machining accuracy of an installation surface of the detector is increased or an error due to the yawing of the detector relative to the scale is corrected after detecting a difference due to the yawing of the detector among readings from the light receiving element arrays provided for the respective tracks in order to detect positions as described in Patent Literature 1.
Moreover, the applicant proposes in Patent Literature 2 that a coil for yawing detection is provided also on the scale side in order to detect the yawing direction of the detector and correct measured values.
Furthermore, the applicant proposes in Patent Literature 3 that a displacement amount in the lateral direction is detected by providing a line sensor in the lateral direction.